1. School of Physics and Materials Science, Nanchang University, Nanchang, Jiangxi 330031, P. R. China
2. Jiangxi Engineering Laboratory for Advanced Functional Thin Films and Jiangxi Key Laboratory for Two-Dimensional Materials, Nanchang University, Nanchang, Jiangxi 330031, P. R. China
3. School of Physics, Shandong University, Shandanan Street 27, 250100 Jinan, P. R. China